Mapping sheep to human brain: The need for a sheep brain atlas

Application and Safety of Externally Controlled Metronomic Drug Delivery to the Brain by an Implantable Smart Pump in a Sheep Model

BACKGROUND AND OBJECTIVES

Intraventricular drug delivery enables the delivery of therapeutics to the central nervous system, while minimizing peripheral drug exposure and toxicity. However, currently used delivery devices cannot be controlled externally to adjust their output during delivery. Here, the authors investigated the performance of a conceptually novel device designed to metronomically deliver a drug to the cerebrospinal fluid in a manner that can be adjusted wirelessly from an external controller.

METHODS

Six sheep were subcutaneously implanted in the shoulder region with a drug delivery pump and a catheter connecting to the brain ventricles. Three groups of 2 sheep received low, medium, and high dosages of metronomic methotrexate (MTX) over several weeks, while kept mobile outdoors in a pen. MTX dosages were adjusted from a wireless external controller, and intraventricular MTX concentrations were measured in regular intervals with an Ommaya reservoir.

RESULTS

Over the course of this 12-week study, sheep showed no signs of toxicity. MTX measurements in the cerebrospinal fluid confirmed that the pump remained responsive to external control and able to deliver drug in an adjustable, metronomic fashion.

CONCLUSION

This implantable pump system enables external control of drug output, so that the resulting intraventricular drug concentrations can continuously be maintained within the therapeutic range.

Characterising a New Sheep Model of Parkinson's Disease Using Unilateral Intracerebral Injection of 6-Hydroxydopamine Into the Substantia Nigra

New therapeutic agents developed for treating neurological disorders are often tested successfully on rodents. Testing in an appropriate large animal model where there is longer lifespan and comparable brain size to humans should improve translational success and is frequently expected by regulatory bodies. In this project, we aimed to establish a novel sheep model of Parkinson's disease as a large-brained experimental model for translational research. Our objective was to create a sheep model of Parkinson's disease by unilaterally infusing the neurotoxin 6-hydroxydopamine into the substantia nigra pars compacta. This approach, previously used to induce parkinsonism in rat and non-human primate models, causes dopaminergic imbalance and induces rotational behaviour in quadrupeds challenged with dopaminergic receptor agonists. In the present sheep study, the mixed dopamine receptor agonist apomorphine, 0.25 mg/kg, and dopamine D2 agonist ropinirole, 0.16 mg/kg, were used to induce rotational behaviour and confirm dopamine depletion. Behavioural signs were then measured and characterised in the field using automated movement tracking with simultaneous video recordings. Post-mortem, the extent of the 6-hydroxydopamine lesions was evaluated through tyrosine hydroxylase immunohistochemistry and quantifying levels of catecholamines (dopamine, 3,4-dihydroxyphenylacetic acid and homovanilic acid) quantified using high-performance liquid chromatography. Our new sheep model of Parkinson's disease using 6-hydroxydopamine is safe and offers a number of regulatory, ethical and financial advantages over non-human primate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine models. It provides a platform to evaluate novel antiparkinsonian agents and medical devices in a large brain with the promise of greater success for translation into clinical application.

Absolute depth-resolved optic axis measurement with catheter-based polarization sensitive optical coherence tomography

Imaging depth-resolved birefringence and optic axis orientation with polarization sensitive optical coherence tomography (PS-OCT) unveils details of tissue structure and organization that can be of high pathophysiologic, mechanistic, and diagnostic value. For catheter-based PS-OCT, the dynamic rotation of the fiber optic probe, in addition to the polarization effects of the system components, complicates the reliable and robust reconstruction of the sample's optic axis orientation. Addressing this issue, we present a new method for the reconstruction of absolute depth-resolved optic axis orientation in catheter-based PS-OCT by using the intrinsic retardance of the protecting catheter sheath as a stable guide star signal. Throughout the paper, we rigorously inspect the retardance and optic axis orientation of the sheath and validate our method by imaging a birefringent phantom with known optic axis orientation. Reconstructing the optic axis orientation of the phantom, placed at different locations around the catheter, we measured an average absolute deviation (AAD) for the mean optic axis orientation over cross-sectional images of 3.28°, even with significant bending stress on the catheter. This corresponds to an almost three-fold improvement compared to our earlier method (optic axis AAD of 9.41°). We finally highlight the capability of our reconstruction with stereotactic catheter-based PS-OCT of a fresh sheep brain.

Long-term hypoxia modulates depolarization activation of BKCa currents in fetal sheep middle cerebral arterial myocytes

Introduction

Previous evidence indicates that gestational hypoxia disrupts cerebrovascular development, increasing the risk of intracranial hemorrhage and stroke in the newborn. Due to the role of cytosolic Ca2+ in regulating vascular smooth muscle (VSM) tone and fetal cerebrovascular blood flow, understanding Ca2+ signals can offer insight into the pathophysiological disruptions taking place in hypoxia-related perinatal cerebrovascular disease. This study aimed to determine the extent to which gestational hypoxia disrupts local Ca2+ sparks and whole-cell Ca2+ signals and coupling with BKCa channel activity.

Methods

Confocal imaging of cytosolic Ca2+ and recording BKCa currents of fetal sheep middle cerebral arterial (MCA) myocytes was performed. MCAs were isolated from term fetal sheep (∼140 days of gestation) from ewes held at low- (700 m) and high-altitude (3,801 m) hypoxia (LTH) for 100+ days of gestation. Arteries were depolarized with 30 mM KCl (30K), in the presence or absence of 10 μM ryanodine (Ry), to block RyR mediated Ca2+ release.

Results

Membrane depolarization increased Ry-sensitive Ca2+ spark frequency in normoxic and LTH groups along with BKCa activity. LTH reduced Ca2+ spark and whole-cell Ca2+ activity and induced a large leftward shift in the voltage-dependence of BKCa current activation. The influence of LTH on the spatial and temporal aspects of Ca2+ sparks and whole-cell Ca2+ responses varied.

Discussion

Overall, LTH attenuates Ca2+ signaling while increasing the coupling of Ca2+ sparks to BKCa activity; a process that potentially helps maintain oxygen delivery to the developing brain.

The prototypical interferonopathy: Aicardi-Goutières syndrome from bedside to bench

Aicardi-Goutières syndrome (AGS) is a progressive genetic encephalopathy caused by pathogenic mutations in genes controlling cellular anti-viral responses and nucleic acid metabolism. The mutations initiate autoinflammatory processes in the brain and systemically that are triggered by chronic overproduction of type I interferon (IFN), including IFN-alpha. Emerging disease-directed therapies aim to dampen autoinflammation and block cellular responses to IFN production, creating an urgent and unmet need to understand better which cells, compartments, and mechanisms underlying disease pathogenesis. In this review, we highlight existing pre-clinical models of AGS and our current understanding of how causative genetic mutations promote disease in AGS, to promote new model development and a continued focus on improving and directing future therapies.

Sheep (Ovis aries) training protocol for voluntary awake and unrestrained structural brain MRI acquisitions

Magnetic resonance imaging (MRI) is a non-invasive technique that requires the participant to be completely motionless. To date, MRI in awake and unrestrained animals has only been achieved with humans and dogs. For other species, alternative techniques such as anesthesia, restraint and/or sedation have been necessary. Anatomical and functional MRI studies with sheep have only been conducted under general anesthesia. This ensures the absence of movement and allows relatively long MRI experiments but it removes the non-invasive nature of the MRI technique (i.e., IV injections, intubation). Anesthesia can also be detrimental to health, disrupt neurovascular coupling, and does not permit the study of higher-level cognition. Here, we present a proof-of-concept that sheep can be trained to perform a series of tasks, enabling them to voluntarily participate in MRI sessions without anesthesia or restraint. We describe a step-by-step training protocol based on positive reinforcement (food and praise) that could be used as a basis for future neuroimaging research in sheep. This protocol details the two successive phases required for sheep to successfully achieve MRI acquisitions of their brain. By providing structural brain MRI images from six out of ten sheep, we demonstrate the feasibility of our training protocol. This innovative training protocol paves the way for the possibility of conducting animal welfare-friendly functional MRI studies with sheep to investigate ovine cognition.

Beyond the surface: how ex-vivo diffusion-weighted imaging reveals large animal brain microstructure and connectivity

Diffusion-weighted Imaging (DWI) is an effective and state-of-the-art neuroimaging method that non-invasively reveals the microstructure and connectivity of tissues. Recently, novel applications of the DWI technique in studying large brains through ex-vivo imaging enabled researchers to gain insights into the complex neural architecture in different species such as those of Perissodactyla (e.g., horses and rhinos), Artiodactyla (e.g., bovids, swines, and cetaceans), and Carnivora (e.g., felids, canids, and pinnipeds). Classical in-vivo tract-tracing methods are usually considered unsuitable for ethical and practical reasons, in large animals or protected species. Ex-vivo DWI-based tractography offers the chance to examine the microstructure and connectivity of formalin-fixed tissues with scan times and precision that is not feasible in-vivo. This paper explores DWI's application to ex-vivo brains of large animals, highlighting the unique insights it offers into the structure of sometimes phylogenetically different neural networks, the connectivity of white matter tracts, and comparative evolutionary adaptations. Here, we also summarize the challenges, concerns, and perspectives of ex-vivo DWI that will shape the future of the field in large brains.

FAST (Flexible Acetylcholine Sensing Thread): Real-Time Detection of Acetylcholine with a Flexible Solid-Contact Potentiometric Sensor

Acetylcholine (ACh) is involved in memory and learning and has implications in neurodegenerative diseases; it is therefore important to study the dynamics of ACh in the brain. This work creates a flexible solid-contact potentiometric sensor for in vitro and in vivo recording of ACh in the brain and tissue homogenate. We fabricate this sensor using a 250 μm diameter cotton yarn coated with a flexible conductive ink and an ACh sensing membrane that contains a calix[4]arene ionophore. The exposed ion-to-electron transducer was sealed with a 2.5 μm thick Parylene C coating to maintain the flexibility of the sensor. The resulting diameter of the flexible ACh sensing thread (FAST) was 400 μm. The FAST showed a linear response range from 1.0 μM to 10.0 mM in deionized water, with a near-Nernstian slope of 56.11 mV/decade and a limit of detection of 2.6 μM. In artificial cerebrospinal fluid, the limit of detection increased to 20 μM due to the background signal of ionic content of the cerebrospinal fluid. The FAST showed a signal stability of 226 μV/h over 24 h. We show that FAST can measure ACh dynamics in sheep brain tissue and sheep brain homogenate after ACh spiking. FAST is the first flexible electrochemical sensor for monitoring ACh dynamics in the brain.

Anatomy of hypothalamic and diencephalic nuclei involved in seasonal fertility regulation in ewes

In this study, we describe in detail the anatomy of nuclei involved in seasonal fertility regulation (SFR) in ewes. For this purpose, the intergeniculate leaflet of the visual thalamus, the caudal hypothalamic arcuate nucleus, and suprachiasmatic, paraventricular and supraoptic nuclei of the rostral hypothalamus were morphometrically and qualitatively analyzed in Nissl-stained serial sections, in the three anatomical planes. In addition, data were collected on calcium-binding proteins and cell phenotypes after immunostaining alternate serial sections for calretinin, parvalbumin and calbindin. For a complete neuroanatomical study, glial architecture was assessed by immunostaining and analyzing alternate sections for glial fibrillary acidic protein (GFAP) and ionized calcium-binding adapter molecule 1 (IBA1). The results showed a strong microglial and astroglia reaction around the hypothalamic nuclei of interest and around the whole 3^rd ventricle of the ewe brain. Moreover, we correlated cytoarchitectonic coordinates of panoramic serial sections with their macroscopic localization and extension in midline sagittal-sectioned whole brain to provide guidelines for microdissecting nuclei involved in SFR.

Harnessing Ultrasound for Targeting Drug Delivery to the Brain and Breaching the Blood–Brain Tumour Barrier

Despite significant advances in developing drugs to treat brain tumours, achieving therapeutic concentrations of the drug at the tumour site remains a major challenge due to the presence of the blood–brain barrier (BBB). Several strategies have evolved to enhance brain delivery of chemotherapeutic agents to treat tumours; however, most approaches have several limitations which hinder their clinical utility. Promising studies indicate that ultrasound can penetrate the skull to target specific brain regions and transiently open the BBB, safely and reversibly, with a high degree of spatial and temporal specificity. In this review, we initially describe the basics of therapeutic ultrasound, then detail ultrasound-based drug delivery strategies to the brain and the mechanisms by which ultrasound can improve brain tumour therapy. We review pre-clinical and clinical findings from ultrasound-mediated BBB opening and drug delivery studies and outline current therapeutic ultrasound devices and technologies designed for this purpose.

The Sheep as a Large Animal Model for the Investigation and Treatment of Human Disorders

Simple Summary

We review the value of large animal models for improving the translation of biomedical research for human application, focusing primarily on sheep.

Abstract

An essential aim of biomedical research is to translate basic science information obtained from preclinical research using small and large animal models into clinical practice for the benefit of humans. Research on rodent models has enhanced our understanding of complex pathophysiology, thus providing potential translational pathways. However, the success of translating drugs from pre-clinical to clinical therapy has been poor, partly due to the choice of experimental model. The sheep model, in particular, is being increasingly applied to the field of biomedical research and is arguably one of the most influential models of human organ systems. It has provided essential tools and insights into cardiovascular disorder, orthopaedic examination, reproduction, gene therapy, and new insights into neurodegenerative research. Unlike the widely adopted rodent model, the use of the sheep model has an advantage over improving neuroscientific translation, in particular due to its large body size, gyrencephalic brain, long lifespan, more extended gestation period, and similarities in neuroanatomical structures to humans. This review aims to summarise the current status of sheep to model various human diseases and enable researchers to make informed decisions when considering sheep as a human biomedical model.